A network is a communications facility that permits a number of workstations, computers or other equipment (hereinafter collectively "computer(s)") to communicate with each other. Portions of a network involve hardware and software, for example, the computers or stations (which individually may comprise one or more central processing units, random access and persistent memory), the interface components, the cable or fiber optics used to connect them, as well as software that governs the access to and flow of information over the network. In networks in which data flow is 100 Mbits/sec. ("Mbps") or higher, the transmission medium is often fiber optics. In networks in which a slower data rate is acceptable, e.g., 10 Mbps, the transmission medium may be coaxial cable or, as is often the case for an Ethernet network, twisted wires.
In a network, network architecture defines protocols, message formats and other standards to which the computers and other equipment, and software must adhere. Most network architectures have adopted a model comprising functional layers in which a given layer is responsible for performing a specific set of functions, and for providing a specific set of services. Thus, the services provided by each layer and the interlayer interfaces can define a network architecture. Protocols define the services covered across a layer interface and the rules followed in the processing performed as a part of that service.
Several organizations have proposed models and standards that have been accepted within the networking community. The International Standards Organization ("ISO"), for example, has proposed a seven layer reference model for computer networking that is called the open systems interconnect ("OSI") architecture. Another set of standards has been promulgated by the Institute of Electrical and Electronics Engineers ("IEEE") set of proposed local area network ("LAN") standards known as IEEE Project 802. This model conforms to the seven-layer OSI model, but directly solely to the lowest two OSI layers, namely the physical layer and the data link layer.
FIG. 1 depicts a network according to the IEEE Project 802 modification to the ISO seven layer model, in which two computers 10, 10' are can communicate data to each other over a physical link medium 20, e.g., cable. Of course, in practice, a network may have many hundreds of computers rather than two.
The bottommost layer 30 in both the ISO and Project 802 model is a physical layer that is concerned with connections between two machines (e.g., computers 10, 10') to allow transmission of bit streams over a physical transmission medium (e.g., cable 20). Thus, physical layer 30 is concerned with types of cabling, cable plugs, connectors, and the like.
As will be described shortly, the present invention is directed to Ethernet networks adhering to the carrier sense multiple access with collision detection ("CSMA/CD") standard. In the 802 model for CSMA/CD, a reconciliation interface 40 defined by a Media Independent Interface ("MII") standard exists for the reconciliation sublayer 40 interface between physical layer 30 and a media access control ("MAC") sublayer 50B. The existing MII signal set provides independent four bit-wide paths for transmission and reception of data, and includes specific "hooks" for link level data flow control. (As used herein, flow control refers to inhibiting access to a network, or one or more links within the network.)
Interestingly, before adoption of MII the MAC standard defined a single carrier sense signal ("CRS") from the physical layer to the MAC that the MAC used to describe the state of the transmit and receive medium. This one CRS signal was used by the MAC transmit process to implement deferral of data transmission, and was by the MAC receive process to frame received data. With introduction of MII, this one CRS signal was decoupled into a CRS signal that again went to the MAC transmit process to implement deferral of data transmission, and into a receive data valid ("RX.sub.-- DV") signal that went to the MAC receive process. Thus, with MII, CRS is used solely by the MAC transmit process
Under MII, data and delimiters are synchronous to the corresponding clock, and two asynchronous media status signals are provided, namely carrier sense ("CRS"), and collision ("COL"). MII provides a two wire serial management interface for control and status gathering, namely management data clock ("MDC"), and management data input/output ("MDIO"). In the OSI seven-layer model, the layer above the physical layer is a data link layer that is responsible for error-free transmission of data frames between network nodes. A data link control protocol describes operation and interfaces of this layer, which must also shield higher layers in the model from concerns about the physical transmission medium.
But in the 802 model shown in FIG. 1, the data link layer is subdivided into MAC layer 50B and an overlying logical link control ("LLC") layer 50A. The media access control sublayer is concerned with access control methods to determine how to control the use of the physical transmission medium. The LLC sublayer 50A is responsible for medium-independent data link functions and allows the network layer 60 above to access LAN services independently of how the network is implemented. According to the 802 architecture, LLC sublayer 50A provides services to network 60 in the same fashion as would a conventional data link protocol in a wide area network.
The MAC sublayer 50B provides services to the overlying LLC sublayer 50A, and manages sharing of the transmission medium among the different stations on the network. A media access management function receives a frame from the data encapsulation function after the necessary control information has been added. Thereafter, media access management is responsible for ensuring physical transmission of the data. The data frame in an Ethernet full-duplex environment has a maximum size of 1,518 bytes.
Several 802 standards exist for MAC sublayer 50B, but only the carrier sense multiple access with collision detection ("CSMA/CD") standard is relevant to the present invention, more specifically the 802.3 standard. The existing 802.3 MAC standard presently contains several mechanisms for performing flow control in a half-duplex environment, including a Deference process, and WatchForCollission and BackOff procedures. CSMA/CD defines data encapsulation/decapsulation and media access management functions performed by MAC sublayer 50B it self, the data encoding/decoding function being performed by underlying physical layer 30.
Physical transmission of the data may be ensured using carrier sensing to defer transmission until the network is clear. In brief, a transmitting station (e.g., computer or user 10) listens or monitors the transmission medium (e.g., cable 20) before transmitting to determine whether another station (e.g., computer or user 10') is currently transmitting a message, e.g., to learn whether the medium is free. Using the services of the physical layer 30, the media access management determines whether the transmission medium (or carrier) is presently being used. If the medium is not being used, media access management passes the data frame to physical layer 30 for transmission. Even after transmission of the frame has begun, media access management continues to monitor the carrier. If the carrier is busy, media access management continues monitoring until no other stations are transmitting. Media access management then waits a specified random time to allow the network to clear and thereafter begins transmission.
But other station(s) having messages to send may all listen simultaneously, discern that the transmission medium appears quiet, and begin to transmit messages simultaneously. The result is a collision and garbled messages. If signal collision is detected, receiving stations ignore the garbled transmission, transmitting stations stop transmitting messages immediately and transmit a jamming signal over the medium. Following collision, each transmitting station will attempt to re-transmit after waiting for a random backoff-delay time period for the carrier to clear. Thus, a station transmitting must listen sufficiently long to ensure that collision has not occurred.
In FIG. 1, network layer 60 concerns the routing of data from one network node to another. It is the role of network layer 60 to route data between network nodes.
Transport layer 70 provides data transfer between two stations at an agreed upon level of quality once a connection is established between the stations. Transport layer 70 selects the particular class of service to be used, monitors transmission to ensure maintained service quality, and advises the stations (or users) if quality cannot be maintained.
Session layer 80 provides services that organize and synchronize a dialogue occurring between stations, and manages data exchange between stations. As such, session layer 80 controls when stations can send and receive data, based upon whether they can send and receive concurrently or alternately.
Presentation layer 90 ensures that information is presented to network users meaningfully, and may provide character code translation services, data conversion, data compression and expansion services.
Application layer 100 provides a mechanism for application processes to access system interconnection facilities for information exchange. The application layer provides services used to establish and terminate inter-user connections, and to monitor and manage the interconnected systems and the resources they employ.
Those skilled in the art will appreciate that the network shown in FIG. 1 may be half-duplex (or shared), or full-duplex. In half-duplex, there is a single shared communications path, and at any given time a station can transmit and monitor, or can receive, but can never simultaneously transmit and receive. In a half-duplex environment, MAC layer 50B uses a CSMA/CD algorithm, e.g., IEEE 802.3, such that a station desiring to use the network to transmit must first listen to learn whether the network is busy. If busy, the station desiring to transmit defers transmission but continues to monitor medium status while waiting for the network to become idle. If idle, the station can begin transmitting. While transmitting, the station continues to monitor the network to ensure that another station does not also begin to transmit at the same time, or transmit in an overlapping fashion, e.g., to ensure there is no collision. Similarly, when a station receives data, it is inhibited from simultaneously transmitting.
As such, the CSMA/CD algorithm provides a useful pacing mechanism that will inhibit or defer access of arrival new data into the half-duplex network until the network is adequately prepared. In addition, a so-called backpressure algorithm is also used in half-duplex environments to inhibit a station from sending data into the network by transmitting to that station. Receipt of the transmission causes the would be transmitting station to believe the network is busy, and to defer or inhibit transmission.
By contrast, in full-duplex, there are transmit and a receive communications paths, and one or more stations may transmit and receive simultaneously. The dual communications channel or path may in fact be multiple wires or cables, or a single wire or cable that simultaneously carries transmit and receive signals in both directions, perhaps using frequency division. Full-duplex networks can provide a higher data rate than half-duplex networks, often 100 Mbps, and are often preferred because of the more rapid communication rate.
Unfortunately, in going to a full-duplex environment, the IEEE 802.3 CSMA/CD MAC half-duplex mechanisms for collision avoidance and for transmission deferral must be abandoned. In a full-duplex environment, a transmitting station that in a half-duplex network would be subject to flow control is permitted to transmit at the same time a receiving station can transmit. The concept of "collisions" or avoiding collisions is meaningless in a full-duplex environment. Further, since a station desiring to transmit over a full-duplex network does not monitor the medium, half-duplex deferral and backpressure flow control procedures are also useless. Thus, the various half-duplex CSMA/CD data pacing mechanisms cannot be used in full-duplex to control or limit access to the network. However, a full-duplex network nonetheless needs some mechanism to inhibit a station from transmitting.
The need for flow control arises because a full-duplex network contains resources that are finite. Networks include devices such as switches, routers, bridges, etc. that connect two network segments together and move data between the segments. These devices typically include buffer memory to accommodate data rate adaptation between links carrying signals at different rates, e.g., perhaps 100 Mbps incoming data presented to a network link capable of handling only 10 Mbps, or to otherwise function when other resources are occupied. While such devices can temporarily store rapidly arriving data during a slower read-out, device memories can readily fill and congest unless some mechanism can halt or slow the entry of new data into the network, or at least the link including the congested device. (While half-duplex networks can also have links with different data rates, the problem is aggravated in full-duplex networks in which considerably more data flows, especially at 100 Mbps.)
Unfortunately, in the prior art there is no flow control mechanism for full-duplex Ethernet networks that avoid such data congestion and resultant data loss, without sacrificing data throughput. Flow control devices implemented in full-duplex networks at the transport layer are slow and inefficient. Transport layer mechanisms can detect a data congestion condition and slow transmission rates. Unfortunately, data throughput will be slower than the rate associated with the congested device using such flow control mechanisms. Thus, data output from a 10 Mbps device receiving data at 100 Mbps will be substantially less than the 10 Mbps rate associated with the device.
Thus, there is a need for a flow control mechanism for a full-duplex Ethernet network. Preferably such mechanism should provide CSMA/CD collision and deferral access to a full-duplex Ethernet network as a flow control method, while maintaining back compatibility with existing networks and standards. Such method should sense when a receiving station or device is about to be (or is) congested with data and should temporarily inhibit a transmitting station or device from transmitting new data until the present data can be handled.
To be compatible with existing networks and equipment and software, such flow control should conform to IEEE 802 standards. Further, to promote its wide use, such flow control mechanism should involve only minor modifications to such standards. Such mechanism should control flow such that the data rate from a slow device coupled to a fast device is the data rate capability of the slow device. Further, while flow control should include a "refresh" mode in which flow control signals are sent relatively often, an option for a time-out mechanism should be provided to deactivate flow control in the event a re-activate flow control signal is somehow not received by a transmitting station.
Further, there is a need in a full-duplex Ethernet network for a MAC layer whose transmitter process and receiver process are each responsive to separately and independently generated signals.
The present invention discloses such a flow control mechanism and such a MAC layer for a full-duplex Ethernet network.